1. Field of the Invention
The present invention relates to an optical transmission apparatus, and more particularly, to an optical transmission apparatus comprising an optical device which externally modulates output light from semiconductor laser and an output unit which connects the output from the optical device to a transmission optical fiber, especially applicable to a wavelength-division multiplexing optical communication system to wavelength-multiplex signal light on a plurality of channels and transmit the multiplexed signal light.
2. Description of Related Art
In the wavelength-division multiplexing optical communication, where optical signals of a plurality of wavelengths are wavelength-multiplexed on the same optical transmission line, it is significant to realize high-performance communication techniques and communication optical information processing techniques, and to realize a downsized and economically-advantageous optical transmission apparatus with stabilized operation.
In wavelength-division multiplexing optical communication, it is significant to suppress wavelength variation of a light source of optical transmission apparatus for a long period. At the present time, the frequencies or wavelengths of respective channels used in communication are precisely determined in spacing of 50 to 100 GHz (about 0.4 to 0.8 nm spacing) by international standardization. For this reason, it is important to set the wavelength of semiconductor laser as the light source to the standard in a stable manner.
The wavelength of the output light of the semiconductor laser as the light source is gradually lengthened during a long-period electrification, and wavelength drift of about 0.1 to 0.2 nm occurs after about 20-year life of the communication apparatus. This value, 25 to 50% of the minimum channel wavelength spacing, 0.4 nm, is an extremely large value, and is inappropriate for the light source of a high density wavelength-division multiplexing optical communication apparatus, based on the above-mentioned international standardization. For example, if the minimum channel spacing is 0.4 nm, the amount of wavelength drift must be reduced to 1/8 of the minimum channel spacing, 0.05 nm, or less.
The wavelength stabilization of the light source may be performed by detecting the wavelength of the light source and feedback controlling the result of detection. However, a wavelength monitor and feedback control, necessary for the wavelength stabilization, disturbs downsizing of transmission apparatus and cost reduction. On the other hand, in current communication systems using long-distance line, the transmission speed is mainly 2.6 to 10 Gb/s. Upon construction of an optical transmission apparatus used in such communication system, a modulator must be an external modulator independent of the semiconductor laser. For the purposes of downsizing the optical transmission apparatus and cost reduction by external modulation, an integrated light source with modulator, where an optical modulator is monolithically integrated with semiconductor laser, is desirable.
As means for increasing conventional semiconductor laser output, a method for improving current-light output characteristic of semiconductor laser by so-called junction down mounting, i.e., providing a device function unit of the semiconductor laser to be opposite to a mounting substrate, is known (The Proceeding of the 1994 Institute Autumn Conference C-312 [All MOVPE growth 1.48 .mu.m deformation MQW-BHLD.]). However, the wavelength stabilization with respect to temperature variation has not been taken into consideration.
The junction down mounting cannot be applied to the above-described light source with modulator where an optical modulator is monolithically integrated with semiconductor laser. That is, to perform modulation in the above-mentioned high-speed transmission, a stray capacitance of the optical modulator must be limited. In the junction down mounting which must limit the capacitance cannot maintain the mechanical strength of the light source. Accordingly, the junction mounting of semiconductor laser where an external modulator is monolithically integrated with the semiconductor laser has been conventionally regarded as impossible.
Note that as a reference about a wavelength-division multiplexing communication system, the Second Optelectronics and Communications Conference (OECC'97) 10C3-1 can be given.